Physicist, b. at Paris, 23 Sept., 1819; d. at Nanteuil, Seine-et-Marne, 18 Sept., 1896. His father, a distinguished physician and professor of medicine in Paris during the Restoration, left him an independent fortune, so that he was able to devote himself to scientific research. He attended Stanislas College and then began to study medicine, but had to abandon it on account of ill-health and travelled for awhile. Then followed Arago's lessons at the Observatory, Regnault on optics at the college of France, and a thorough study of his brother's notebooks of the courses at the Ecole Polytechnique. In 1839 he became interested in the new photography and succeeded in getting permanent pictures by the daguerreotype. Foucault came to consult him about this work and became associated with him in their epoch-making experiments in optics, showing the identity of radiant heat and light, the regularity of the light vibrations, and the validity of the undulatory theory. Just as they were ready to develop the experimentum crucis (see FOUCAULT) overthrowing the emission theory, they parted company and worked independently.

Fizeau was the first to determine experimentally the velocity of light (1849). He used a rotating cogwheel and a fixed mirror several miles distant; light passed between two teeth of the wheel to the distant mirror and then returned. If the wheel turned fast enough to obscure the reflection, then the reflected beam struck a cog. The time it took the wheel to move the width of one tooth was then equal to the time it took the light to travel twice the distance between the wheel an the mirror. He also experimented successfully to show that the ether is carried along by moving substances, since light travels faster through a stream of water in the direction of its motion than in the opposite direction. In his measurements of vanishingly small distances, such as the expansion of crystals, he made use of the extremely small and very regular wave-length of light. His addition of a condenser in the primary circuit of the induction coil increased the effectiveness of this device considerably. On the recommendation of the Academy of Sciences he was awarded the Grand Prix (10,000 francs) of the Institute in 1856. He was elected a member of the Academy of Sciences in 1860, an a member of the Bureau des Longitudes in 1878. He received the decoration of the Legion of Honour in 1849 and became officer in 1875. In 1866 the Royal Society of London awarded him the Rumford Medal. Cornu says of him: "He was a practical and convinced Christian and did not hide that fact." In the presidential address before the academy (Comptes Rendus, 1879), Fizeau calls attention to "the dignity and independence of natural science as well as to its limits of action, preventing it from interfering in philosophic or social questions, and not permitting it to put itself in opposition to the noble emotions of the heart nor to the pure voice of conscience". Most of his published works appeared in the "Comptes Rendus" and in the "Annales de physique et de chimie". A few of the titles are: "Sur la dageurréotypie"; "Sur l'interférence entre deux rayons dans le cas de grandes différences de marche"; "Vitesse de la lumière"; "Interférence des rayons calorifiques"; "Réfraction différentielle"; "Vitesse de l'électricité"; "Dilatation des cristaux".